Chevron World Challenge third-round leaderboard:
-17 T Woods (US) -13 G McDowell (NI) -9 P Casey (Eng) -7 H Mahan (US), S O'Hair (US), L Donald (Eng), R McIlroy (NI) -6 I Poulter (Eng), N Watney (US)
Woods is showing a glimpse of the form that won 14 majors
Tiger Woods maintained his four-shot lead over Graeme McDowell with one round left of the Chevron World Challenge he is hosting in California.
The world number two fired a 68 to reach 17 under as US Open champion McDowell matched him to go 13 under.
England's Paul Casey (69) is nine under with Hunter Mahan, Sean O'Hair, Luke Donald and Rory McIlroy seven under.
Woods, who has won four times in the charity event at Sherwood Country Club, is chasing his first title this season.
The 14-time major champion faltered with an early bogey via the water on the par-five second as playing partner McDowell closed to within one stroke after birdies at the second and third.
But the swing changes Woods has been implementing with new coach Sean Foley appear to be paying dividends as he leads a tournament for the first time since the Australian Masters in November 2009.
He hit three straight birdies from the fifth followed by three more after the turn, including two in his last three holes.
He struck a superb approach to within a foot at the par-four last, sparking huge roars from watching gallery.
Woods missed last year's tournament as his off-course sex scandal exploded around him and missed the year before as he recovered from knee surgery.
Casey was five under for his round through seven holes before a double bogey at the short eighth pegged him back.
Donald and McIlroy both had disappointing rounds of 73, with McIlroy suffering double bogeys on both the ninth and 18th holes.
American Dustin Johnson is propping up the exclusive 18-man field after amassing a third-round 80, reminiscent of the 82 he carded after leading McDowell by three going into the final round of the US Open in June.